The present invention relates to a precipitation hardening type stainless steel for spring, which has an excellent processability, including good forming and punching workabilities, because of its reduced level of work hardening when cold worked and, which exhibits, when age-hardened, a high strength and other desirable spring performances which are substantially isotropic.
Illustrative of typical known stainless steel for spring one can mention the following two species:
(a) work hardening type stainless steel represented by SUS 301 steel, and;
(b) precipitation hardening type stainless steel represented by 17-7PH steel.
The work hardening type stainless steel (a) above is based upon the utilization of the hardness of the martensite itself which has been induced by cold working. Accordingly, in order to achieve sufficient properties for a spring material, such as a high spring limit value, high fatigue limit and high hardness, an intensive cold working is required so as to form appreciable amounts of martensite. Because the formation of martensite is adversely affected by high temperatures, the cold working must be carried out at a low rate to avoid an increase of the temperature of the material, leading to a low productivity. Inevitable variations in the compositions from charge to charge results in variations in the stability of the austenite phase, and this fact makes it difficult to form a constant amount of martensite by a constant amount of cold working, leading to variations in the properties of the product. Moreover, the intensive cold working required to achieve the high strength is expensive. In a case wherein an EH material having a hardness of at least Hv 490 as prescribed in JIS G4313 should be prepared a cold working with a rolling reduction of at least 50% is required, and the material so cold worked has a poor forming workability and gives rise to a problem in that when such a material is fabricated to a spring element by punching, the punching tools are unduly worn.
The 17-7PH steel (b) mentioned above is a precipitation hardening type steel and therefore, difficulties as involved is SUS 301 are not encountered in order to achieve a high strength. However, this steel has a structure of a substantial austenite phase, at the state of as having been solution treated, which phase must be converted to a martensite phase by cold working. Accordingly, there are difficulties in the manufacturing process as is the case with SUS 301. Furthermore, in order to achieve a final hardness of at least Hv 490 after being age hardened, a cold working with a rolling reduction of at least 40% is required, and the thus cold worked material has a hardness of at least about Hv 400 exhibiting poor forming and punching workabilities. Moreover, the 17-7PH steel contains an appreciable amount of .delta.-ferrite due to its relatively high content of Al, and in consequence the yield in the hot working steps is reduced, rendering the manufacturing cost expensive.
As discussed above, the known types of stainless steel for spring suffer from conflicting limitations in that an attempt to achieve an increased final hardness requires an intensive cold rolling, resulting in an unduly high hardness and poor forming and punching workabilities at the state of having been cold worked, while an attempt to improve the forming and punching workabilities of the material as cold worked results in insufficient final hardness after being aged. Furthermore, the attainable final hardness of a spring element made from the known types of stainless steel for spring has been still unsatisfactory in comparison with difficulties involved in the manufacturing process.
We previously developed a stainless steel for spring which has an improved workability and processability when compared with those of SUS 301 and 17-7PH and which exhibits a martensite structure at the state of having been solution treated or at the state of having been solution treated and then slightly cold worked. We proposed such a steel in Japanese Patent Application No. 51-131610, assigned to the same assignee for "Stainless Steel for Spring Having Improved Forming Workability and Processability and Exhibiting Improved Increase in Hardness by Aging" (see Japanese Patent Laid-open Specification No. 53-57114, published on May 24, 1978).
The subject matter of this Japanese Patent Application No. 51-131610 is a stainless steel comprising, in % by weight, not more than 0.03% of C, 0.5 to 2.5% of Si, not more than 3.0% of Mn, 5.0 to 9.0% of Ni, 14.0 to 17.0% of Cr, 0.5 to 2.5% of Cu, 0.3 to 1.0% of Ti, not more than than 1.0% of Al and not more than 0.03% of Ni, the balance being Fe and unavoidable impurities, the contents of Mn, Ni, Cr, Cu, Si, Ti and Al being further adjusted so that the value of A defined by the equation (i): EQU A=0.70.times.(Mn%)+1.times.(Ni%)+0.60.times.(Cu%)+0.76.times.(Cu%)-0.63.tim es.(Al%)+20.871 (i)
is less than 39.0, the value of Cr equivalents/Ni equivalents defined by the equation (ii); ##EQU2## is not more than 2.7, and the value of H defined by the equation (iii): EQU H=4.times.[(Ti%)-5.times.(C%+N%)]+4.times.[(Al%)-3.times.(N%)]+2.8.times.(S i%)+1.times.(Cu%) (iii)
is within the range between 5.5 to 8.5. We further found that the material having the elements adjusted in the manner as described above may be cold worked with a rolling reduction of 5 to 50% prior to the age hardening step so that a good forming workability and an enhanced ability of being age hardened as well as a good elongation after age hardened may be achieved. The process was proposed in Japanese Patent Application No. 51-131611, assigned to the assignee of this application, for "Process for Producing Stainless Steel for Spring Having Improved Forming Workability and Toughness and Exhibiting Enhanced Ability of Being Age Hardened" (see Japanese Patent Laid-open Specification No. 53-57115, published on May 24, 1978). The inventions claimed and disclosed in the above-mentioned Japanese Patent Applications make much account of the forming workability before aging as well as the strength and toughness after aging, and respectively relate to a stainless steel for spring exhibiting an enhanced ability of being age hardened and a process for the production of such a stainless steel for spring. The steel has a martensite structure, and therefore, not to detract from its workability the carbon content is held at a low level.
Leaf spring elements, including snap rings, bellevills springs, spring washers, toothed washers and the like, are generally fabricated by punching suitable materials. Accordingly, the material for such spring elements should have a moderately reduced hardness before aging. Since the punched piece is frequently formed into the final element by bending, the material for spring should also possess a good forming workability. Furthermore, it is widely practiced to form a thin material for spring into various shapes of a small size by bulging, drawing and/or bending thereby to manufacture a miniaturized spring element whose reduced durability and strength are compensated by its shape. Again, a good forming workability is required here. On the other hand, the material for spring should possess a high strength and other enhanced spring characteristics after aging. As to these requirements the spring material described in Japanese Patent Application No. 51-131610 is fairly satisfactory. Nevertheless, a further improvement is still desired.